This invention relates in general to devices for providing simultaneous transmission of analog communication and sporadic bursts of digital data over a bandwidth limited analog channel, and in particular to such devices that remove from the band available for the analog communications only those frequencies necessary to accomplish the data transmissions and only during the data transmissions.
A need exists for a way to transfer, from time to time, bursts of digital data over an analog channel simultaneously with analog communications without degrading the fidelity of the analog communications except only slightly during the bursts of digital data. Such a need exists in the art of telephone communications, particularly with the introduction of a feature commonly called "caller identity delivery" also known as "caller i.d." (hereinafter sometimes referred to as "CID"). This feature provides a user with certain identifying information of a caller, such as telephone number and/or the name of the caller. This identifying information, i.e. data, is transmitted digitally from a telephone company to its users' telephone equipment via the same voice frequency channels used for voice communication for display on their respective equipment. In the case where the user is not using the telephone when a caller rings, the CID information is transferred between the first and second ring signals. The user can then view the information to decide whether to answer the telephone.
The techniques for transmitting digital data over an analog channel such as a voice frequency channel are well known in the art. For example transmission of the digital data can be, and is most commonly, accomplished using a technique called "FSK" or frequency shift keying. For another example, digital data can also be transmitted over such channels by a technique called "PSK" or phase shift keying. Both of these techniques utilize a portion of the frequency spectrum available on the channel.
A conflict can arise when a user has the CID feature and also has a telephone feature commonly called "call waiting." (The combination of the two features is commonly called "caller i.d. on call waiting" hereinafter sometimes referred to as "CID/CW"). If a user's equipment is connected to another party, the call waiting feature alerts the user to the presence of a waiting call by a distinctive audible indication. The user can then elect to receive the waiting call by a known keying operation. The conflict arises whenever the user is telephoned by a caller while the user is engaged in a telephone conversation with another party and the caller's CID information is transmitted to the user's receiver. The CID transmission takes place almost automatically after the user is alerted to the new call so that this information can be used to help decide whether to take the new call. Since both the analog signals of the conversation between the user and the other party, and the digital signals corresponding to the caller's CID information are transmitted over the same analog channel, there can be audible interference between the two while the CID information is being transmitted. The user and the other party involved in the conversation can be subjected to annoying and uncomfortable bursts of sound generated by the CID data transmission.
A heretofore system that has addressed this CID/CW problem uses a technique of muting all sound to the conversing parties while the caller i.d. information is being transmitted. This system has the advantage of providing an error free channel for the CID data communication, but it has a significant disadvantage in that it causes the conversation to be muted for the three to four seconds it takes to transmit the CID information.
Another heretofore proposed system for CID/CW involves using a spread spectrum signal for CID data transfer mixed with the voice signal. This system is too expensive and too complicated to implement. Moreover, the spread spectrum signal will be heard as noise to the user reducing the clarity of the voice communication during CID transfer.
The present invention avoids the muting without creating noise and is able to transmit CID information over a channel with no interruption in voice communication and without subjecting the conversing parties to the aforesaid annoying and uncomfortable burst of sound. It has the further advantage of not reducing the band of frequencies available on the channel for voice communications except only slightly during the brief CID data transmissions. It also provides an error free path for the data communications.
Other advantages and attributes are either discussed in, or can be gleaned from a reading of, the text hereinafter.
Previous methods of achieving simultaneous transmission of speech and data have been disclosed in U.S. Pat. Nos. 4,523,311, 4,512,013 and 4,280,020. These inventions multiplex the data and the analog signal thereby continuously degrading the quality of the analog signal.